What is a polar vortex?

The American side of Niagara Falls on January 9th 2014. The massive falls partially froze over during a polar vortex event. Credit:Michael Muraz.

When temperatures plunged into the sub-zero range for much of the country in early 2014, the term "polar vortex" became a household expression. Sure, it sounds like it could be some sort of alien death-ray or an extremely powerful washing machine, but what does it have to do with cold weather?

The polar vortex is an area of low pressure—a wide expanse of swirling cold air—that is parked in polar regions. We are interested in the one up north, since that’s the one that has been causing all this wild weather. Its not a new thing—this low-pressure system is almost always up there.

Sometimes this low-pressure system, full of cold Arctic air, strays a little bit too far from home. Part of it can break off and migrate southward, bringing all of that cold air with it. Just like that, areas as far south as Florida get to experience their own little taste of life in the Arctic.

We actually want a strong polar vortex to stay warm?

The breaking off of part of the vortex is what defines a polar vortex event. But it actually occurs when the vortex is weaker, not stronger. That might sound weird—but it actually makes sense. Normally, when the vortex is strong and healthy, it helps keep a current of air known as the jet stream traveling around the globe in a pretty circular path. This current keeps the cold air up north and the warm air down south.

But without that strong low-pressure system, the jet stream doesn’t have much to keep it in line. It becomes wavy and rambling. Put a couple of areas of high-pressure systems in its way, and all of a sudden you have a river of cold air being pushed down south along with the rest of the polar vortex system.

Regular, strong polar vortex with fairly stable jet stream (left) compared to the early 2014 weak polar vortex with a detached low pressure system over the United States and wavy jet stream (right).

The fountain in front of the National Museum of the American Indian in Washington, DC is frozen solid on on January 8th 2014 during the polar vortex event. Credit:Phil Osrtoff.

That’s what happened in early 2014. The polar vortex suddenly weakened, and a huge high-pressure system formed over Greenland. The high-pressure system blocked the escape of all that cold air in the jet stream, and allowed part of the polar vortex to break off and move southward. Places as far south as Tampa, Florida experienced the wrath of this wandering polar vortex. Most of Canada and parts of the Midwestern United States had temperatures colder than Alaska at the height of this cold snap!

It's important to remember that not all cold weather is the result of the polar vortex. While the polar vortex is always hanging out up north, it normally minds its own business. It takes pretty unusual conditions for it to weaken or for it to migrate far south, and other things can cause cold arctic air to travel our way, too.

And what are high- and low-pressure systems?

Air might feel like nothing to you and me, but it is actually super heavy. In fact, the pressure caused by all those gases in the atmosphere stacked on top of each other creates a great deal of pressure-about 14.7 pounds pressing on every inch of our body (learn more here). We don't notice it because we are used to it.

But not all areas have the same exact air pressure. Some areas have more pressure than their surroundings, and some areas have less.

A low-pressure system

Those that have less pressure are called low-pressure systems. Low-pressure systems "suck" air into them because nature wants everything to have equal pressure. By doing this, they generally create winds and undesirable weather.

A high-pressure system

High-pressure systems, on the other hand, have more air pressure than their surroundings. That means they are constantly pushing air away from them into the areas that have lower pressure. They are often times associated with clear blue skies.

A polar vortex is simply a semi-permanent, massive low-pressure system that hovers over the poles of our planet.